1. Field of the Invention
The present invention relates generally to a wireless communication system, and more particularly, to an apparatus and a method for retaining a beam direction of beamforming in a wireless communication system.
2. Description of the Related Art
A transmitter of a wireless communication system is capable of transmitting an electric signal using an antenna, and a receiver of the same system is capable of receiving the electric signal from the transmitter over a radio channel. The Friis equation is typically used, at the receiver, as a gain model of the receive signal using gains of the transmit and receive antennas.
FIG. 1 is a diagram illustrating a transmitter and a receiver in a wireless communication system. When a transmitter 110 has an antenna gain of Gt and a receiver 120 has an antenna gain Gr, the Friis equation is provided as set forth in Equation (1) below.
                              P          r                =                              P            t                    ⁢                                                    G                t                            ⁢                              G                r                            ⁢                              λ                2                                                    16              ⁢                                                          ⁢                              π                2                            ⁢                              d                2                                                                        (        1        )            
In Equation (1), Pr denotes a power of a receive signal, Pt denotes a power of a transmit signal, Gt denotes the antenna gain of the transmitter, Gr denotes the antenna gain of the receiver, λ denotes a wavelength, and d denotes a distance between the transmitter and the receiver.
Equation (1) is applicable in a free space. Accordingly, in an actual system, some modifications can be made according to characteristics of a radio channel. Equation (1) signifies that the power received at the receiver is proportional to the antenna gains of the transmitter and the antenna gains of the receiver.
FIGS. 2A and 2B are diagrams illustrating power radiation patterns of an antenna in a wireless communication system. FIG. 2A illustrates the radiation pattern of the power of an isotropic antenna, and FIG. 2B illustrates the radiation pattern of the power of a directional antenna. Referring to FIG. 2A, the power radiated from the isotropic antenna is circular or spherical. By contrast, the power radiated from the directional antenna of FIG. 2B is in the form of a fan or a cone. When the power value radiated from the antennas of FIGS. 2A and 2B is equal, the directional antenna of FIG. 2B concentrates the power in a particular direction and the resulting signal of FIG. 2B can be propagated farther than the signal transmitted via the isotropic antenna of FIG. 2A. The reception based on the antenna directivity is the antenna gain value in the Friis equation of Equation (1). Specifically, as the beam range of the antenna becomes narrower, the gain of the antenna increases. However, since the power of the beam is concentrated in a particular direction and range, its serviceable area reduces. Specifically, the antenna gain is typically inversely proportional to the service area.
The performance of the wireless communication system can be enhanced in accordance with the radiation pattern of the signal, as described above. The technique of forming the radiation pattern of the intended signal is commonly referred to as beamforming. Beamforming increases the transmit and receive gains by applying directivity to the beam using multiple antennas, thereby raising the signal reception strength of a specific receiver. Specifically, the beamforming technique depends on how the antenna gain and the service direction and range are set.
FIG. 3 is a diagram illustrating beams of a base station adopting the beamforming technique in a wireless communication system. Referring to FIG. 3, the base station covers a plurality of sectors (i.e., 1, 2 and 3), and forms a plurality of directional beams in the respective sectors. When beamforming is applied, the signal range is narrow. Accordingly, in order to service one base station cell or sector, it is necessary to utilize a plurality of beams per antenna in the sector. For example, in FIG. 3, Beams #1-3 are formed in Sector #1. As the base station performs beamforming of the narrow range, a user station can attain better channel state.
The user station selects the best beam from the beams of the base station and selects a new beam according to its motion or movement. The narrower the beam width, the more frequently a new beam is selected. When the user station also adopts beamforming, the best channel status is attained when the beam direction of the base station aligns with the beam direction of the user station. However, unlike the stationary base station, the user station is mobile, and a direction of the device with respect to the base station is easily changeable. As a result, the beam direction of the user station can be frequently misaligned with the beam direction of the base station.
FIGS. 4A to 4D are diagrams illustrating beam directions of a base station and a user station in a wireless communication system, according to an embodiment of the present invention.
Referring first to FIG. 4A, a beam direction for transmission and reception at a base station 410 aligns with a beam direction for transmission and reception at a user station 420. When the beam direction of the base station 410 faces the user station 420, the user station 420 lies within a beam range of the base station 410. In order to improve reliability and efficiency of data transmission and reception, the user station 420 also forms a beam toward the base station 410. When the beam directions of the base station 410 and the user station 420 are aligned as shown in FIG. 4A, optimum communication quality is attained.
Typically, beam training is adopted to straighten the beam direction between the base station and the user station. Beam training is divided into a downlink and an uplink. In the downlink, the base station allocates a particular sequence for beam training to beam training reference signals of a specific direction, and transmits the reference signals as the beam of the specific direction. Hence, the user station informs the base station of a particular code value of the reference signal having the best communication quality amongst one or more beam training reference signals received from the base station. Thus, the aligned beam direction of the base station and the user station can be determined.
In the uplink, the user station allocates a particular code for beam training to beam training reference signals of a specific direction, and transmits the reference signals as the beam of the specific direction. The base station informs the user station of a particular code value of the reference signal having the best communication quality amongst the beam training reference signals.
Beam training is adequate for stationary base stations and user stations. However, when the user station frequently moves in a short period of time, it is difficult to apply the beam alignment method using beam training. FIGS. 4B, 4C and 4D illustrate this shortcoming in detail. Specifically, FIGS. 4B, 4C and 4D illustrate misalignment of the beam directions according to the motion and the movement of the user station 420. Referring to FIG. 4B, as the user station 410 is tilted, the beam directions of the base station 410 and the user station 420 are misaligned in accordance with the amount of tilt. Referring to FIG. 4C, when the user station 420 rotates, the beam directions of the base station 410 and the user station 420 are misaligned in accordance with the amount of rotation. As the user station 420 moves as shown in FIG. 4D, the beam directions of the base station 410 and the user station 420 are misaligned in accordance with the amount of movement.
As shown in FIGS. 4B, 4C and 4D, the beam directions of the base station 410 and the user station 420 can misalign according to the motion or movement of a user. When misaligned, the communication quality of the base station 410 and the user station 420 is greatly deteriorated. Further, since the motion or movement of the user is frequent over a relatively short time, it is hard to overcome this deterioration with the typical beam training method. More specifically, in order to detect any change in the beam direction of the user station 420, the base station 410 needs to conduct the beam training on a very short cycle. Since accurate information about a change in the beam direction of the user station 420 is required, control messages are frequently exchanged, which increases system overhead.